What a question. You can't answer that in a sentence or two. And I doubt you would accept any facts anyway.
It's hard to prove to you how things are better in the Union than out of it, because you can always speculate and do "what ifs" in relation to the UK not being part of the Union. However, just about every indicator says that being a member of the Union is, and has been, very beneficial for any nation involved. No, I'm not going to spend my evening finding a bunch of numbers trying to convince you. Do your own research or revel in your ignorance.
However, I'll shortly try to give a starting push in your research. The easiest way to see if the EU is economically beneficial is to see how countries in it fare compared to those out of it. I have a book on European Economics with statistics, but not at hand so I'll have to get back to you with specific numbers. However, the general and obvious trend was that the members of the EEC had a higher yearly economic growth than the members of the EFTA. It's the reason why the majority of the EFTA members, including the UK who had founded the EFTA as a reaction to the EEC, eventually decided it was better to just enter the EEC.